1. Field of the Invention
The present invention relates to a nonvolatile memory device. In particular, the invention relates to a random-access memory including memory cells having magnetic tunnel junctions (MTJ).
2. Description of the Background Art
In recent years, an MRAM (Magnetic Random-Access Memory) device has been of great interest as a new-generation nonvolatile memory device. The MRAM device is a nonvolatile memory device in which a plurality of thin-film magnetic elements formed in a semiconductor integrated circuit are used to store data in nonvolatile manner, and the thin film magnetic elements are each randomly accessible. In particular, recently it has been announced that an MRAM device is remarkably improved in performance by employment of the thin-film magnetic element using a magnetic tunnel junction (MTJ) as a memory cell.
Generally, in the case where data is read from a memory cell which is used as a storage element of the nonvolatile memory device, electric current flowing through a tunneling magneto-resistance element (TMR) forming the storage element or an end-to-end voltage of the TMR can be measured so as to indirectly measure the resistance value of the TMR and thereby read data.
On the other hand, development has been carried out for producing a cell configuration of the MRAM device through a simple process, like that for a cell configuration of a DRAM (Dynamic Random-Access Memory).
Specifically, while a memory cell of a common MRAM device adopts a cell configuration having word lines for writing in addition to word lines for reading, memory cells according to the spin-injection method have recently been proposed as memory cells for which it is unnecessary to provide word lines for writing, as disclosed for example in Japanese Patent Laying-Open Nos. 2005-011907, 2004-111904 and 2005-092912.
The memory cell of the spin-injection method differs from a currently-used MRAM device in terms of how data is written. For a memory cell of the currently-used MRAM device, a method is employed of allowing current to flow through lines (including write word line) adjacent to a TMR element to generate magnetic fields and thereby reverse magnetization. In contrast, for the memory cell of the spin-injection method, a method is employed according to which current is applied directly through a TMR element to reverse magnetization of the TMR element. The direction in which the current flows is changed to switch magnetization of a free layer to parallel or antiparallel magnetization with respect to a fixed layer. In this respect, this is called the spin injection method since magnetization is reversed by the function of spin-polarized electrons in the current. In this way, a simple cell configuration can be achieved without the necessity of particularly providing word lines for writing to memory cells of the MRAM device.
However, regarding an MRAM device of the spin injection method, data is written to a memory cell by reversing magnetization using a current applied directly to a TMR element as described above and therefore, depending on an amount of a current applied then for reading the data, the stored data could be inverted as it is read, namely a write error could occur.